This specification relates to generating selection criteria for selecting content.
The Internet provides access to a wide variety of resources. For example, video and/or audio files, as well as web pages for particular subjects, are accessible over the Internet. Access to these resources presents opportunities for content items, such as advertisements (or other content items) to be provided with the resources or with search results that identify the resources. For example, a web page can include “slots” (i.e., specified portions of the web page) in which advertisements (or other content items) can be presented. These slots can be defined in the web page or defined for presentation with a web page, for example, in a separate browser window. Advertisements or other content items that are presented in slots of a resource are selected for presentation by a content distribution system.